This answer only concerns support theorems.

First, some non-microlocal support theorems (to put more readers on the map):

The last two of these results use microlocal techniques, so reading the papers can be illuminating for the microlocal perspectives as well.
For microlocal aspects, I suggest taking a look at The geodesic ray transform on Riemannian surfaces with conjugate points and references therein.
In the absence of conjugate points, if the X-ray transform for geodesics in a neighborhood of a geodesic $\gamma$ vanishes, the wavefront set of the unknown function doesn't meet the conormal bundle of $\gamma$.
(This is true in all dimensions. This talk I heard is a nice summary of what is known.)
The X-ray transform fails to an elliptic Fourier integral operator if there are conjugate points, but the attenuated X-ray transform still recovers singularities if there are no more than two conjugate points per geodesic.

I haven't found a good reference for the fact that one can recover $N^*\Gamma\cap WF(f)$ from the knowledge of $Xf(\gamma)$ for $\gamma\in\Gamma$, where $\Gamma$ is an open set of lines in a Euclidean space and $N^*$ denotes the conormal bundle.
This result and many others are covered in the book "Microlocal Analysis and Integral Geometry" by Stefanov and Uhlmann, but the book is still in progress.
That book will certainly be an up-to-date reference material for microlocal techniques for ray transforms.